From Marco Jez, Added an unregister of the GraphicsContextImplement on unload.
From Marco Jez, Added an unregister of the GraphicsContextImplement on unload.


ghub/osg,ntoand/osg,omega-hub/osg,koshman86/osg,stmh/osg,ntoand/osg,ntoand/osg,stmh/osg,omega-hub/osg,XiaoCC/osg,XiaoCC/osg,ghub/osg,stmh/osg,artoolkit/osg,artoolkit/osg,artoolkit/osg,ghub/osg,artoolkit/osg,stmh/osg,koshman86/osg,ghub/osg,koshman86/osg,koshman86/osg,omega-hub/osg,stmh/osg,XiaoCC/osg,XiaoCC/osg,omega-hub/osg,ntoand/osg,XiaoCC/osg